Method for manufacturing multilayer head core

ABSTRACT

In a metal-insulator alternate type multilayer head core usable for magnetic sound recording and reproducing, metallic component layers are connected to each other via one or more metallic interconnecting layers passing locally through insulating component layers in order to provide high effective permeability with minimized eddy current loss. In manufacturing, a metal-insulator alternate type multilayer construction including the metallic interconnecting layer or layers is subjected to sintering in order to avoid intermetal separation which might be otherwise caused by cracking in the insulating layers.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 894,607, filed Apr. 7, 1978,now U.S. Pat. No. 4,242,711, granted Dec. 30, 1980.

BACKGROUND OF THE INVENTION

The present invention relates to an improved multilayer head core and amethod for manufacturing the same, and more particularly relates toimprovements in a multilayer head core used for magnetic heads inmagnetic sound recording and reproducing devices, magnetic videorecording and reproducing devices, and the like, and in a method formanufacturing same.

It is in general required for the magnetic material for head cores tohave high magnetic permeability and high abrasion resistance accompaniedby reduced eddy current loss in the high frequency range of the electriccurrent.

However, relatively high permeability of such material leads toincreased eddy current loss in the high frequency range when suchmaterial is used for head cores while resulting in lower effectivepermeability.

In order to avoid such lowering in effective permeability, it isproposed to superimpose a plurality of thin layers of the magneticmaterial such as permalloy and a plurality of insulating material layersalternately to each other to obtain multilayer head cores. For thispurpose, the conventional magnetic alloy material such as permalloy isfirst subjected to cold working to produce thin layers of head coreshape, and a plurality of such thin layers are bonded together byadhesive with insulating material layers interposed therebetween toobtain a head core structure.

Recently, the performance requirements for the head core have becomehigher and higher and new magnetic alloy materials such as sendust andalperm alloys are spotlighted in the possible use to the head cores.These new alloys, indeed, have excellent characteristics as magnetichead core materials, but low workability of these alloys makes it verydifficult to work the material alloys into thin layers with lowmanufacturing cost.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multilayer headcore having high effective permeability with relatively small eddycurrent loss in the high frequency range.

It is another object of the present invention to provide a method formanufacturing such an excellent multilayer head core even from difficultto workable magnetic materials such as sendust and alperm alloys whichare quite unsuited for cold working.

In accordance with the present invention, the multilayer head coreincludes a plurality of metallic component layers and a plurality ofinsulating component layers superimposed to each other in an alternatefashion, the metallic component layers being connected to each other viaat least a metallic interconnecting layer.

In accordance with the manufacturing method of the present invention, aplurality of metallic material layers and a plurality of insulatingmaterial layers are superimposed and secured to each other in analternate fashion, the powdery metallic material layers are connected toeach other via at least an interconnecting powdery metallic materiallayer, both the metallic and insulating material layers are compacted toform a compressed multilayer construction, and sintering is thereafterapplied to the compressed multilayer construction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side sectional view of one embodiment of the multilayer headcore in accordance with the present invention,

FIGS. 2A through 2C are side sectional views for showing operationalsteps of one embodiment of the method for manufacturing the head coreshown in FIG. 1,

FIGS. 3A through 3C are side sectional views for showing operationalsteps of another embodiment of the method for manufacturing the headcore shown in FIG. 1,

FIGS. 4A and 4B are side sectional views for showing operational stepsof the other embodiment of the method for manufacturing the head coreshown in FIG. 1,

FIGS. 5A and 5B are side sectional and explanatory plan views of a headcore test piece used in examples in which manufacturing was carried outin accordance with the present invention,

FIG. 6 is a graph for showing the relationship between the effectivepermeability of the test piece and the frequency of the electric currentapplied to the test pieces obtained in various examples in which cendustalloy is used for metallic component layers,

FIG. 7 is a side sectional view of a head core test piece used inexamples with which the head core test piece of the present invention iscompared.

FIG. 8 is a graph for showing the relationship between the effectivepermeability and the cross-sectional surface ratio in percent of theinsulating component layers in a test piece at a fixed frequency of theelectric current applied to the test piece, cendust alloy being used formetallic component layers,

FIG. 9 is a graph for showing the relationship between thecharacteristic increase in effective permeability and thecross-sectional surface ratio in percent of the insulating componentlayers in a test piece at a fixed frequency of the electric currentapplied to the test piece, sendust alloy being used for metalliccomponent layers,

FIG. 10 is a graph for showing the relationship between the effectivepermeability of the test piece and the frequency of the electric currentapplied to the test pieces in various examples in which alperm alloy isused for metallic component layers,

FIG. 11 is a graph for showing the relationship between the effectivepermeability and the cross-sectional surface ratio in percent of theinsulating component layers in a test piece at a fixed frequency of theelectric current applied to the test piece, alperm alloy being used formetallic component layers, and

FIG. 12 is a graph for showing the relationship between thecharacteristic increase in effective permeability and thecross-sectional surface ratio in percent of the insulating componentlayers in a test piece at a fixed frequency of the electric currentapplied to the test piece, alperm alloy being used for metalliccomponent layers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to manufacture head cores of excellent characteristics,particularly from hard-to-work magnetic alloy materials, various testshave been conducted and as a result the application of powder metallurgycomes out to be effective.

In the application of powder metallurgy to the manufacture of laminatedhead cores, following steps are selectively available: powdery metallicmaterials and powdery insulating materials are superposed to one anotherin an alternate fashion, such laminated materials are compacted to forma compressed multilayer construction, and the compressed multilayerconstruction is subjected to sintering; each of the powdery metallic andinsulating material layers is pre-compacted before another layer issuperposed thereon, such layers are laminated to form a multilayerconstruction, and the multilayer construction is subjected to sintering;and such powdery materials are pressed to obtain a number of separatelayers, the separate layers are superimposed one another to form amultilayer construction, and the multilayer construction is pressed andcompacted and then is subjected to sintering. As the insulating materialis required to have sufficient resistance against head applicationduring the sintering process, it is quite impossible to use popularinsulating materials such as synthetic resins or rubbers.

As a substitute, it is proposed to use such insulating materials asalumina, magnesia and silica glass. In this connection, however, thesintering temperatures for such oxides are relatively high and in arange from 1,700° to 3,000° C. whereas the sintering temperatures of themagnetic alloys are relatively low and generally in a range from severalhundreds to 1,500° C.

Therefore, when sintering is carried out at the sintering temperaturesof the alloys, oxide powders in the insulating layers cannot be wellsintered. Only mechanical coupling is attained between the oxide powdersin the insulating layers and between the oxide powder layers (insulatinglayers) and the metal layers. This weak coupling leads to seriousproblems such as development of cracks in the oxide powder layers andseparation between the metal layers.

On the contrary, when sintering is carried out at the sinteringtemperature of the insulating materials, metallic powders in the alloylayers start to melt so that the multilayer construction cannot preserveits shape. For this reason, there is no other way but to employ thesintering temperature of the alloys in order to carry out sintering ofthe multilayer construction.

This solution is still accompanied by another trouble. Thermalexpansivity of the above-described oxides is relatively small being in arange from 0.1˜4×10⁻⁶ /° C. whereas the thermal expansivity of theabove-described alloys is relatively large being in a range from 5 to30×10⁻⁶ /° C. This big difference in the thermal expansivity betweenboth materials tends to cause development of cracks in the insulatinglayers in the multilayer construction during cooling after thesintering. Development of such cracks in the insulating layersinevitably induces separation between the alloy layers.

Elimination of any trouble mentioned above has resulted in the followingembodiments.

An embodiment of the multilayer head core in accordance with the presentinvention is shown in FIG. 1, in which the head core 1 is comprised of aplurality of metallic component layers 3 and a plurality of insulatingcomponent layers 5 which are alternately superimposed to each other. Themetallic component layers 3 are securely inter-connected in one body toeach other via a metallic interconnecting layer 7 which runs across theextending direction of the metallic component layers 3 in order tointernally embrace the component layers 3 and 5. That is, when thecomponent layers 3 and 5 take the form of thin circular discs or thinrings, the interconnecting layer 7 is formed on the outer peripheralportion of the head core 1. However, when the component layers 3 and 5take the form of the thin rings, the interconnecting layer 7 may beformed on the inner peripheral portion, too. In this case, aninterconnecting layer 7 may be formed on the inner peripheral portiononly. In a further variant, the interconnecting layer 7 may fully fillthe center portion of the head core 1. It is only required that themetallic component layers 3 alternately sandwiching the insulatingcomponent layers 5 are securely interconnected in one body to each othervia the interconnecting layer or layers 7.

The present invention is applicable to head cores of any magneticmetallic materials, but is most advantageously applied to ones ofhard-to-work magnetic metallic materials such as sendust and alpermalloy materials. Herein, the term "hard-to-work magnetic metallicmaterials" means the magnetic metallic materials which are relativelyeasily subjected to hot working but are relatively hardly subjected tocold working. Here, the sendust alloy is defined as an alloy consistingof 0.001 to 8.0 wt.% (percent by weight) of one or more elements chosenfrom a group composed of 0.01 to 6.0 wt.% of Nb, 0.1 to 5.0 wt.% of Mo,0.1 to 5.0 wt.% of Ti, 0.1 to 7.0 wt.% of Cr, 0.1 to 5.0 wt.% of V, 0.1to 7.0 wt.% of Ni, 0.05 to 6.0 wt.% of Cu, 0.1 to 5.0 wt.% of W, 0.1 to5.0 wt.% of Ta, 0.1 to 5.0 wt.% of Ge, 0.1 to 5.0 wt.% of Hf, 0.1 to 5.0wt.% of Zr, 0.01 to 3.0 wt.% of rare earth element or elements, 0.1 to5.0 wt.% of Mn, 0.001 to 0.5 wt.% of P, 0.01 to 5.0 wt.% of Y, 0.001 to0.5 wt.% of B, 0.1 to 5.0 wt.% of Ti and 0.1 to 5.0 wt.% of Pb; 3 to 8wt.% of Al; 3 to 12 wt.% of Si; and remaining percent by weight of Fe.The alperm alloy consists of 16 percent by weight of Al and the balanceof Fe. The sendust alloy as well as alperm alloy is known as a magneticmaterial having high initial and maximum permeabilities, extremely highhardness and excellent abrasion resistance.

The insulating layers 5 are made of materials having sufficient thermalresistance at the sintering temperature of the magnetic metallicmaterial used for the metallic component layers 3. For example, oxidessuch as alumina, magnesia and silica glass can be advantageously used inthe present invention.

The interconnecting layer 7 may be made of a metallic material eithersame (or similar) to or different from that used for the metalliccomponent layers 3. It is only required for the metallic material ormaterials for both layers 3 and 7 that metallic powder particles shouldbe strongly bonded to each other as a result of sintering, given no illphysical and chemical influences upon the resultant metallic layersduring actual use of the head core and form no bar to smooth working ofthe head core. In one example, the component layers may be made ofsendust alloy powder and the interconnecting layers are made of ironpowder.

One embodiment of the method for manufacturing the head core of theabove-described construction in accordance with the present invention isshown in FIGS. 2A through 2C, in which the head core to be manufacturedis assumed to have a column shape. A cylindrical mould 11 closed at thebottom is prepared for press. Metallic material powder is dispersed andfilled in the mould 11 to a prescribed depth in order to form the firstmetallic powdery layer 13a as shown in FIG. 2A. Next, an annular mask 14is placed on the top surface of the first metallic powdery layer 13a.Preferably, the outer diameter of the mask 14 is equal to the innerdiameter of the cylindrical mould, the inner diameter of the mask 14 isequal to the outer diameter of the later described first insulatinglayer and the thickness of the mask 14 is equal to the depth of thefirst insulating layer. After emplacement of the mask 14, insulatingmaterial powder is dispersed and filled in the space defined by the mask14 to a prescribed depth, i.e. the thickness of the mask 14, in order toform the first insulating powdery layer 15a as shown in FIG. 2B. Afterremoval of the annular mask 14, the metallic material powder is againdispersed and filled in the mould 11 to a prescribed depth in order toform the second metallic powdery layer 13b. By this filling, the annularspace previously occupied by the annular mask 14 is filled with themetallic material powder, also. Thus, the first and second metallicpowdery layers 13a and 13b are interconnected to each other via acylindrical metallic powdery layer 17 as shown in FIG. 2C whilesandwiching the first insulating powdery layer 15a.

By repeating the above-described process, a multilayer intermediateconstruction is formed in the cylindrical mould 11 in which a pluralityof flat insulating powdery layers and a plurality of flat metallicpowdery layers are alternately superimposed to each other while thelatter being mutually interconnected in one body to each other by thecylindrical metallic powdery layer. After the above-describedsuperimposition is complete, a suitable press such as the knownhydrostatic press is applied utilized to apply pressure to themultilayer intermediate construction within the mould 11 in order toobtain a multilayer construction usable for the magnetic head core shownin FIG. 1.

A modified embodiment of the manufacturing method in accordance with thepresent invention is shown in FIGS. 3A through 3C, in which a likecylindrical mould 11 for press is employed. In this case, formation ofthe flat metallic powdery layers and the cylindrical metallic powderylayer is substantially similar to that imployed in the precedingembodiment. Metallic material powder is dispersed and filled in themould 11 to a prescribed depth in order to form the first metallicpowdery layer 23a shown in FIG. 3A. In this case, no mark is used forbuilding up the insulating layer. As a substitute, insulating materialplates are separately shaped by suitable preparatory press. Aninsulating material plate 25a is placed in position on the top surfaceof the first metallic powdery layer 23a as shown in FIG. 3B. Aftercorrect emplacement of the first insulating material plate 25a, themetallic material powder is again dispersed and filled in the mould 11to a prescribed depth in order to form the second metallic powdery layer23b. By this filling, the annular space surrounding the first insulatingmaterial plate 25a is filled with the metallic material powder also.Thus, the first and second metallic powdery layers 23a and 23b areinterconnected to each other via a cylindrical metallic powdery layer 27as shown in FIG. 3C while sandwiching the first insulating materialplate 25a.

By repeating the above-described process, a multilayer intermediateconstruction is formed in the cylindrical mould 11 in which a pluralityof flat insulating material plates and a plurality of flat metallicpowdery layers are alternately superimposed to each other while thelatter being mutually interconnected in one body to each other by thecylindrical metallic powder layer. After the above-describedsuperimposition is complete, suitable press such as the knownhydrostatic press is applied to the multilayer intermediate constructionwithin the mould in order to obtain a multilayer construction usable forthe magnetic head core shown in FIG. 1. The press may be applied aftereach time filling of the metallic material powder.

A further modified embodiment of the manufacturing method in accordancewith the present invention is shown in FIGS. 4A and 4B, in which a likecylindrical mould 11 for press is employed. In this case, a multilayerintermediate construction is separately shaped by suitable preparatorypress. This multilayer intermediate construction includes a plurality ofmetallic material layers 33 and a plurality of insulating materiallayers 35 superimposed and bonded in one body to each other in analternate fashion. The multilayer intermediate construction is placed inposition in the mould as shown in FIG. 4A while leaving a cylindricalspace therearound. Next, the metallic material powder is dispersed andfilled in the above-described cylindrical space to the level of the topsurface of the multilayer intermediate construction. Thus, the metallicmaterial layers 33 in the multilayer intermediate construction areinterconnected to each other via a cylindrical metallic powdery layer 37as shown in FIG. 4B. After the above-described filling is complete, asuitable press such as the known hydrostatic press is employed in orderto obtain a multilayer construction usable for the magnetic head coreshown in FIG. 1.

For pressing of the multilayer intermediate construction, the knownhydrostatic press employing a rubber press is advantageously utilized.In this case, it is recommended to repeat the press for several timesand, more advantageously, employ intermediate annealing between pressesin order to obtain a compressed multilayer construction of extremelyhigh density.

Next, the compressed multilayer construction is subjected to sintering.The sintering conditions such as sintering temperature and sinteringtime are chosen in reference to the physical characteristics of thematerial metallic powder. For example, when Sendust alloy is used as thematerial metal, the sintering temperature should be in a range from 900°to 1,350° C. and the sintering time in a range from 30 to 600 minutes.As the environmental gas during sintering, highly vacuum air of 10⁻⁴ Hg.or lower, hydrogen gas having a dew point of -30° C. or lower, or inertgas having a dew point of -35° C. or lower is preferably used.

In the case of using alperm alloy as the material metal, the sinteringtemperature should preferably be in a range from 800° to 1,400° C. andhe sintering time in a range from 30 to 600 minutes. Hydrogen gas of adew point of -30° C. or lower or inert gas of a dew point of -35° C. orlower may preferably used for the environmental gas at sintering.

When the sintering temperature is lower than 800° C., no sintering canbe carried out ideally. Any sintering temperature exceeding 1,400° C.may cause melting of the component layers. When the dew points of theabove-described gases exceed the above-described limits, possibleoxidization of the material seriously lowers magnetic characteristics ofthe head core obtained.

Sintering time shorter than 30 minutes assures no successful sinteringwhereas that longer than 600 minutes is economically inadvantageous.

By employment of the sintering, particles of the metallic powder orpowders are sintered and bonded in one body to each other. Thanks tosuch fortified bonding between the metallic particles, no separationoccurs between the metallic component layers even when any crackdevelops in the insulating component layers during sintering, coolingafter sintering and later-staged grinding and abrasing.

The following examples are illustrative of the present invention but arenot to be construed as limiting the same.

EXAMPLE 1.

A sendust alloy consisting of 9.7 wt.% of Si, 5.5 wt.% of Al, 1.0 wt.%of ti and remaining percent by weight of Fe was used for the metallicmaterial powder whereas silica (SiO₂) powder was used for the insulatingmaterial. A cylindrical metallic mould closed at the bottom was used forpressing with an outer and inner annular masks for formation of theinsulating powdery layers. Both powders were filled alternately withapplication of press at each filling. The obtained compressed multilayerconstruction has a cylindrical shape of 6 mm. inner diameter and 10 mm.outer diameter and includes four metallic powdery layers and threeinsulating powdery layers alternately superimposed to each other.

The compressed multilayer construction was then subjected to sinteringat 1,250° C. for 160 minutes within hydrogen gas having a dew point of-40° C. or lower. The test piece, i.e. the multilayer construction, soobtained is shown in FIGS. 5A and 5B, which has a culindrical shape ofthe following particulars.

Inner diameter r: 6 mm.

Outer diameter R: 10 mm.

Thickness of each metallic component layer 3. Ta:0.3 mm.

Thickness of each insulating component layer 5. Tb:0.05 mm.

Thickness of each metallic interconnecting layer 7. Tc:0.1 mm.

Thickness of the multilayer construction 10. T:1.35 mm.

Frequency of the electric quantity to be applied to the above-describedmultilayer construction test piece was changed from 300 Hz. to 100 KHzand the measured values of the effective permeability are shown with acurve A in FIG. 6.

EXAMPLE 2.

Like in Example 1, a cylindrical test piece, i.e. a multilayerconstruction, as shown in FIG. 7 was prepared from similar metallicmaterial powder and insulating material powder with only exception thatno mask was used for formation of the insulating powdery layer. Theparticulars of this multilayer construction test piece were quite samewith those of the test piece used in Example 1 but this multilayerconstruction 20 lacks in the metallic layer or layers interconnectingthe metallic component layers. In other words, the metallic componentlayers 3 were isolated from each other by the intervening insulatingcomponent layers 5. Similar electric test was applied to this test pieceand the measured values of the effective permeability are shown with acurve B in FIG. 6.

EXAMPLE 3.

Using the metallic material powder alone used in Example 1, a likecylindrical multilayer construction test piece was prepared. Theparticulars of the test piece are almost same with those of the testpiece used in Example 1 with exceptions that no insulating componentlayer was included and the total thickness of the test piece was 1.2 mm.Similar electric test was applied to this test piece and the measuredvalues of the effective permeability are shown with a curve C in FIG. 6.

The results in Examples 1 through 3 are numerically shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Effective permeability                                                        Frequency in KHz                                                              0.3           1      3        10   30     100                                 ______________________________________                                        Example 1                                                                             14,000    9,600  5,900  3,000                                                                              1,350  750                               Example 2                                                                             15,000    9,800  6,100  3,200                                                                              1,400  800                               Example 3                                                                              3,500    3,000  2,500  1,800                                                                              1,100  680                               ______________________________________                                    

It is clearly learned from FIG. 6 that the lowering in the effectivepermeability of the test piece in Example 1 (present invention) fromthat of the test piece in Example 2 is extremely small whereas theeffective permeability of the test piece in Example 1 (presentinvention) is remarkably higher than that of the test piece in Example3.

EXAMPLE 4.

A number of multilayer construction test pieces were prepared in amanner substantially similar to that employed in Example 1 with onlyexception that the thickness Tc of each of the metallic interconnectinglayers 7 was changed from 0 to 2 mm. Electric current of 1 KHz frequencywas applied to the test pieces and the measured values of the effectivepermeability are shown in FIG. 8, in which the cross-sectional surfaceratio ρ in percent of the insulating component layers to the totalcross-sectional surface area of the test piece is taken on the abscissa.

The above-described result is numerically shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        ρ     Effective permeability                                              ______________________________________                                         0        3,500                                                               13        5,000                                                               28        6,500                                                               47        8,000                                                               72        9,000                                                               92        9,600                                                               ______________________________________                                    

As is clear from these representations, the effective permeability ofthe test piece is in a range from 3,000 to 4,000 when the surface ratioρ is zero, i.e. when no insulating component layer is included in thetest piece. However, whenthe surface ratio ρ exceeds 50, i.e. when thecross-sectional surface ratio of the metallic interconnecting layerfalls short of 50%, the effective permeability of the test piece becomes8,000 or higher. Further, when the surface ratio ρ exceeds 70, i.e. whenthe cross-sectional surface ratio of the metallic interconnecting layerfalls short of 30%, the effective permeability of the test piece becomes9,000 or higher. Therefore, despite the presence of the metallicinterconnecting layer between the metallic component layer sandwichingthe insulating component layer, increase in eddy current loss can besuccessfully avoided in order to obtain very high effective permeabilityby decreasing cross-sectional surface ratio of the metallicinterconnecting layer or layers.

In addition to the foregoing analysis, the relationship between thecross-sectional surface ratio ρ and the characteristic increase ineffective permeability is shown in FIG. 9. Here, the characteristicincrease in effective permeability is defined by the following formula.##EQU1## M₁₀₀ ; Effective permeability when the ratio ρ is 100. M_(O) ;Effective permeability when the ratio ρ is 0.

M_(x) ; Effective permeability when the ratio ρ is x.

From FIG. 9, it is clear that, as concerns sendust alloy, the effectivepermeability is acceptable when the ratio ρ is 30 or larger except for100, the effective permeability being 6,600 when the ratio ρ is 30.

EXAMPLE 5.

Instead of the sendust alloy used in Example 1, the alperm alloyconsisting of 16 wt.% of Al and 84 wt.% of Fe was used for the metallicmaterial powder and a compressed maltilayer construction was prepared ina manner substantially similar to that employed in Example 1. Sinteringwas carried out at 1,250° C. for 120 minutes within hydrogen gas of adew point -35° C. The obtained cylindrical multilayer construction testpiece has two metallic component layers of 0.3 mm. thickness for eachand an insulating component layer of 0.05 mm. thickness, total thicknessof the test piece being accordingly 0.65 mm. The cross-sectional surfaceratio ρ in percent of the insulating component layers was 94.

Electric test was applied to the test piece in a manner similar to thoseemployed in Examples 1 through 3 and measured values of the effectivepermeability are shown with a curve D in FIG. 10.

EXAMPLE 6

Using the materials employed in Example 5, a test piece was preparedwhich includes, just like in Example 2, no metallic interconnectinglayer. Result of the electric test is shown with a curve E in FIG. 10.

EXAMPLE 7

Using the alperm alloy employed in Example 5, a test piece was preparedwhich includes, just like in Example 3, neither metallic interconnectinglayers nor insulating component layer. Result of the electric test isshown with a curve F in FIG. 10.

The results in Examples 5 through 7 are numerically shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Effective permeability                                                        Frequency in KHz                                                              0.3           1      3        10   30     100                                 ______________________________________                                        Example 5                                                                             4,600     2,400  1,180  510  200    80                                Example 6                                                                             5,000     2,600  1,340  560  230    88                                Example 7                                                                             1,480      820    420    190  89     38                               ______________________________________                                    

As is clear from these representation, the lowering in the effectivepermeability of the test piece in Example 5 (present invention) fromthat of the test piece in Example 6 is extremely small whereas theeffective permeability of the test piece in Example 5 (presentinvention) is remarkably higher than that of the test piece in Example7, in the case of alperm alloy also.

EXAMPLE 8

Just like in Example 4, electric current of 1 KHz frequency was appliedto test pieces in which the thickness Tc of each of the metallicinterconnecting layers 7 was changed from 0 to 2 mm. The measured valuesof the effective permeability are shown in FIG. 11, in which thecross-sectional surface ratio ρ of the insulating component layers tothe total cross-sectional surface area of the test piece is taken on theabscissa.

The above-described result is numerically shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        ρ      Effective permeability                                             ______________________________________                                        0            820                                                              20           900                                                              33.99      1,400                                                              54         2,000                                                              70.3       2,200                                                              87.76      2,300                                                              ______________________________________                                    

As is clear from these representations, the effective permeability ofthe test piece is about 800 when the surface ratio ρ is zero, i.e. whenno insulating component layer is included in the test piece. However,when the surface ratio ρ exceeds 50, i.e. when the cross-sectionalsurface ratio of the metallic interconnecting layer falls short of 50%,the effective permeability of the test piece becomes 1,800 or higher.Further, when the surface ratio ρ exceeds, i.e. when the cross-sectionalsurface ratio of the metallic interconnecting layer falls short of 30%,the effective permeability of the test piece becomes 2,200 or higher.Therefore, despite the presence of the metallic interconnecting layerbetween the metallic component layers sandwiching the insulatingcomponent layer, increase in eddy current loss can be successfullyavoided in order to obtain very high effective permeability bydecreasing cross-sectional surface ratio of the metallic interconnectinglayer or layers, in the case of alperm alloy also.

The relationship between the characteristic increase in effectivepermeability and the cross-sectional surface ratio ρ is shown in FIG. 12in which it is clear that, as concerns alperm alloy, the effectivepermeability is acceptable when the ratio ρ is 40 or larger except for100, the effective permeability being 1,700 when the ratio is 40.

From the results of the various tests, it is learned in general that theeffective permeability is acceptable when the cross-sectional surfaceratio ρ in percent of the insulating component layers is 40 or largerexcept for 100.

In accordance with the present invention, separation of the metalliccomponent layers during the manufacturing and/or the later-stagedprocesses can be successfully avoided without causing any substantialincrease in eddy current loss.

We claim:
 1. A method for manufacturing a multilayer head corecomprising:preparing a compressed multilayer construction which includesa plurality of metallic powdery layers, a plurality of insulating layerssuperimposed with said metallic powdery layers in an alternate fashionand at least an interconnecting metallic powdery layer locallyconnecting said metallic powdery layers to each other, and subjectingsaid compressed multilayer construction to sintering at a temperaturesufficient to sinter said metallic powdery layer but insufficient tomelt the same, whereby said interconnecting metallic powdery layer addsmechanical strength to said construction and at least reduces crackingdue to differential thermal expansion.
 2. A method for manufacturing amultilayer head core as claimed in claim 1 in which the step of saidpreparing includesdispersing and filling metallic material powder to aprescribed depth in a given mould in order to form the first metallicpowdery layer, locally covering the top surface of said first metallicpowdery layer with at least a mask, dispersing and filling insulatingmaterial powder to a prescribed depth in said mould to form the firstinsulating powdery layer, removing said mask, dispersing and fillingmetallic material powder to a prescribed depth in said mould in order toform the second metallic powdery layer, repeating the above-describedprocess in order to form a multilayer intermediate construction of aprescribed design in said mould, and subjecting said multilayerintermediate construction to pressure.
 3. A method for manufacturing amultilayer head core as claimed in claim 1 in which the step of saidpreparing includes,dispersing and filling metallic material powder to aprescribed depth in a given mould in order to form the first metallicpowdery layer, emplacing an insulating material plate on the top surfaceof said first metalling powdery layer, the effective cross-sectionalsurface area of said plate being smaller than that of said mould,dispersing and filling metallic material powder to a prescribed depth insaid mould in order to form the second metallic powdery layer, repeatingthe above-described process in order to form a multilayer intermediateconstruction of a prescribed design in said mould, and subjecting saidmultilayer intermediate construction to press.
 4. A method formanufacturing a multilayer head core as claimed in claim 1 in which thestep of preparing includes,emplacing a multilayer intermediateconstruction including alternately superimposed metallic material layersand insulating material layers in a given mould, the effectivecross-sectional surface area of said construction being smaller thanthat of said mould, dispersing and filling metallic material powder intospaces in said mould unoccupied by said construction, and applying pressto materials in said mould.
 5. A method for manufacturing a multilayerhead core as claimed in claim 1 in which said sintering is carried outat a temperature in a range from 800° to 1,400° C.
 6. A method formanufacturing a multilayer head core as claimed in claim 1 in which saidsintering is carried out for a period in a range from 30 to 600 minutes.7. A method as claimed in claim 1 in which said sintering is carried outwithin an environmental gas chosen from a group consisting of hydrogengas having a dew point of -30° C. or lower, vacuum air of 10⁻⁴ mm. Hg orhigher and inertgas having a dew point of -35° C. or lower.
 8. A methodfor manufacturing a multilayer magnetic head core, comprising the stepsof:providing a stack comprising a plurality of alternating parallellayers of insulating material and compressed powdered particles of a lowductility metallic high permeability magnetic alloy material; disposingparticles of a powdered metallic material around the edges of saidinsulating layers and extending between said magnetic layer; compressingsaid layers and metallic particles to form a compressed multilayerconstruction; and heating said compressed multilayer construction at atemperature sufficient to sinter said particles to each other, saidtemperature being below the melting point of said magnetic material,whereby the sintered particles of a powdered metallic material aroundthe edges of said insulating layers and extending between said magneticlayers add mechanical strength to said multilayer magnetic head core andat least reduce cracking due to differential thermal expansion.
 9. Themethod of claim 1, 2, 3, 4, 5, 6, 7 or 8, wherein the metallic powderylayer locally connecting said metallic powdery layers to each otherencloses the insulating layers within the multilayer construction.